1. Field of the Invention
This invention relates to the field of coated carbon fibers and to the field of carbon fiber composite materials.
2. Description of the Prior Art
It is now known that fibers produced from certain materials may have exceptional mechanical properties, properties which are many times greater than those of the same material produced in bulk form. Because of their form, however, such fibers have limited utility. To increase the utility, it is known to combine many such fibers in a suitable matrix material to form a composite material having a good overall combination of properties. Such a composite material is typlified by the material commonly known as fiberglass which consists of glass fibers in an organic resin matrix. Among the fibers which have been used in composites are glass fibers, boron fibers, ceramic fibers, metal fibers and carbon fibers. An overview of the fiber composite area is presented in the book entitled "High Modulus Fibers and Composites" by F. S. Galasso, published by Gordon and Breach, Science Publishers, Inc., New York, N.Y., 1969.
Carbon fibers have received a great deal of attention since they have a low density and high modulus. Such fibers are typically produced by heating an organic precursor fiber at a high temperature to first carbonize the fiber and then graphitize the carbonized fiber. Commercial carbon fibers have diameters of 5-10 microns and can be quite long; e.g., 10 feet or more. Such carbon fibers can be used in conjunction with a wide variety of organic matrices to produce composite materials. The specific details of the carbon fiber preparation and matrix material do not form a part of the present invention. Rather the invention is directed at a method for increasing the electrical resistance of the carbon fibers and their resistance to oxidation and corrosion so that when the fibers are incorporated in an organic matrix they will have high stability; and in the event of a fire and subsequent release into the air will not cause electrical short circuits.
It has been known in the prior art to form silicon carbide on various substrates by chemical vapor deposition. Silicon carbide filaments have been produced by using heated filaments as a substrate. Since the intent is to produce a high strength filament of silicon carbide, the precursor filaments are of small diameter relative to the finished filament diameter; e.g., 10%.
Limited work has been done using carbon fibers as a starting filament. For example, the article "Elevated Temperature Strength of Silicon Carbide on Carbon Filaments" by K. D. McHenry and R. E. Tressler in J. of Composite Materials V9 (January 1975) pp. 73-76 describes filaments formed on a starting carbon fiber of 0.001 in diameter. On this was deposited a pyrolytic graphite layer about 0.0001 thick, followed by silicon carbide. The resultant fiber was 0.004 inches in diameter. In this situation, the carbon core constituted about 7.5% of the final fiber area with silicon carbide making up the balance.
Similar work is described in "Improvement of Manufacturing Methods for the Production of Low Cost Silicon Carbide Filament" by H. DeBolt and V. Krokonis, AFML-TR-73-140 December 1972. This report is incorporated herein by reference for its teachings relating to specific methods of silicon carbide deposition.
SiC coated carbon fibers may also be advantageously combined with metal matrices. The SiC surface is more resistant to attack by molten metal than is the uncoated carbon surface.